The invention relates to communications system using optical networks.
In general, the information conveyed in such systems is constituted by binary data in the form of pulses clocked at a determined clock frequency. Binary value is represented by pulse amplitude. Initially, the pulses are in electrical form, and then they are converted into an optical signal obtained by modulating the power (or the amplitude) of an optical carrier wave.
The advantage of optical systems is that the optical fibers that make up transmission links enable data to be transmitted at much higher rates than the rates possible on electrical lines.
In analogous manner, a difference of the same order of magnitude exists between the data-rate capacities of optical-domain systems and those of electrical-domain circuits. One of the problems posed is thus to design electrical-to-optical conversion interfaces that enable the data-rates to be matched.
One solution consists in providing time-division multiplexing whereby a plurality of electrical signals to be transmitted are taken synchronously in parallel manner and are then transmitted in serial manner over the network, in optical form. Advantageously, the pulses forming the transmitted data are compressed to make best possible use of the passband of the optical network.
In a particular case applicable in ATM-type networks, such time-division multiplexing is performed on packets of successive bits of the various signals rather than on the bits of said signals.
However, the interface that performs these compression, packet-forming and parallel-to-serial conversion operations must be compatible with the performance of electronics, and must not be expensive to implement.
To solve those problems, the invention proposes methods of concatenating and deconcatenating binary information, which methods make it possible to perform compression, packet-forming, and decompression entirely optically, by making use of the spectrum dimension of the optical domain so as to avoid creating interference noise.
To this end, the invention provides a method of concatenating binary information contained in successive time windows of an amplitude-modulated synchronous input signal, said windows being of duration w and of period T not less than twice the duration w, wherein, with g being a number lying in the range 2 to T/w, said method consists in particular in:
forming g converted input signals obtained by amplitude modulating respective ones of g optical carrier waves having different wavelengths, each converted input signal presenting, within said time windows, amplitude modulation as a function of said input signal; and
forming a multiplex signal made up of a combination of g delayed signals obtained by applying delays to said converted input signals, the delays being such that any two consecutive delayed signals are offset in time by said duration w of the time windows.
Thus, the invention makes it possible to concatenate the binary information from the input signal in groups of g bits or in groups of g blocks of successive bits.
In a first possibility, said converted input signals are combined before said delays are applied to them, by making use of the differences in their wavelengths.
In another possibility, said delays are applied to said converted input signals before they are combined.
Although these two possibilities are functionally equivalent in principle, the first possibility is less costly to implement, as explained below.
In the case when said input signal is a binary signal of period T that is to be initially compressed in a ratio T/w, and then concatenated in groups of g bits with a period equal to g.T, the converted input signals are obtained by amplitude modulating the q carrier waves by a sampling clock signal constituted by pulses of frequency 1/T, of constant amplitude, and of width equal to said duration w so as to form g optical sampling signals, and then by respectively amplitude modulating said optical sampling signals as a function of the input signal.
In another case, the input signal is a binary signal already organized into blocks of binary data, the blocks being of time width equal to w and being spaced apart with a period T. The binary signal is thus constituted by pulses of frequency not less than 2/T and including blocks of binary data contained in respective ones of successive time windows of duration w and of period T. It is then possible to concatenate the blocks in groups of g blocks with block group period equal to g.T, by making provision for the converted input signals to be obtained by amplitude modulating the carrier waves by a selection signal constituted by pulses of frequency 1/T, of constant amplitude, and of width equal to said duration w, so as to form g optical selection signals, and then by respectively amplitude modulating said optical selection signals as a function of the input signal.
The invention further provides a concatenation circuit for implementing the above-defined concatenation method. The circuit comprising:
first means for forming g converted input signals obtained by amplitude modulating respective ones of g optical carrier waves having different wavelengths, each converted input signal presenting, within said time windows, amplitude modulation as a function of said input signal; and
second means for forming a multiplex signal made up of a combination of g delayed signals obtained by applying delays to said converted input signals, the delays being such that any two consecutive delayed signals are offset in time by said duration w of the time windows.
By means of the above-described concatenation method, it is theoretically possible to form packets of binary information, i.e. groups containing a large number of bits. In practice, the circuits serving to perform wavelength conversion are of passband that is limited in terms of wavelength, which means that the number g must also be limited. In addition, for certain implementations, increasing the number of carriers gives rise to a proportional deterioration in the signal-to-noise ratio.
In another aspect of the invention, that problem can be solved by forming the packets in a plurality of successive concatenation steps, each of which uses the above-described method.
More precisely, the invention also provides a method of forming packets grouping together binary information contained in successive time windows of a synchronous amplitude-modulated input signal, said windows being of duration w and of period T not less than twice the duration w, said method comprising:
a first concatenation step consisting in applying the above-described concatenation method to said input signal, by taking a determined value g1 for g, so as to form a first multiplex signal; and
at least one subsequent concatenation step consisting in applying the above-described concatenation method to said first multiplex signal considered as an input signal whose time windows are of period g1.T, each of which time windows contains g1 successive time windows of the input signal.
The invention also provides a packet-forming circuit for implementing the above-defined method. The circuit comprising a plurality of concatenation circuits connected in cascade.
The invention also provides a method making it possible to perform the operation that is the inverse of the above-described concatenation operation. More precisely, the invention provides a method of deconcatenating packets of binary information contained in an amplitude-modulated synchronous received signal, said packets being of period Ta, and each of them including gxe2x80x2 successive blocks of binary information, said method consisting in particular in:
breaking down said received optical signal into gxe2x80x2 converted block signals obtained by amplitude modulating respective ones of gxe2x80x2 optical carrier waves having different wavelengths, each converted block signal presenting amplitude modulation as a function of the binary information contained in blocks of period Ta and belonging to successive packets of the received optical signal; and
forming a deconcatenated signal obtained by combining and delaying said converted block signals so as to form the delayed blocks of period Ta/gxe2x80x2.
The invention finally provides a communications system comprising an optical network, at least one transmitter terminal and at least one receiver terminal, wherein at least one of said transmitter terminals includes packet-forming circuits as defined above having outlets coupled to an optical circuit for time-division multiplexing packets, and wherein at least one of said receiver terminals includes an optical circuit for time-division demultiplexing packets.